Polymerization-inhibiting composition, polymerization inhibitor and method for inhibiting polymerization

ABSTRACT

The invention relates to a polymerization inhibitor comprising at least one compound (a) selected from the group consisting of a compound having an NO radical in its molecule and a precursor compound capable of forming an NO radical, and a phosphorus-containing compound (b), wherein a weight ratio of the compound (a) to the phosphorus-containing compound (b) is 1:10 to 100:1. 
     Specifically, the invention provides a method for inhibiting the polymerization of at least one monomer (c) selected from the group consisting of a conjugated diene, an aromatic vinyl, an ethylenically unsaturated nitrile and an α-olefin, comprising causing the polymerization inhibitor to coexist with the monomer (c), and a polymerization-inhibiting composition comprising the polymerization inhibitor and the monomer (c).

TECHNICAL FIELD

The present invention relates to techniques for inhibiting thepolymerization of a monomer such as a conjugated diene, aromatic vinyl,ethylenically unsaturated nitrile or α-olefin, and more particularly toa polymerization-inhibiting composition, a polymerization inhibitor anda polymerization-inhibiting method for inhibiting the occurrence ofunfavorable premature polymerization in various operating steps such asthe production, purification, storage, shipment, preparation and use ofsuch a monomer, a mixture of the monomers or a hydrocarbon mixturecontaining the monomer.

BACKGROUND ART

A monomer such as a conjugated diene, aromatic vinyl, ethylenicallyunsaturated nitrile or α-olefin is easy to cause prematurepolymerization in various operating steps such as the production,purification, storage, shipment, preparation and use thereof. Theoccurrence of the premature polymerization causes contamination of themonomer with a polymer, increase of viscosity, gelling, loss inreactivity, etc. In addition, by the premature polymerization, a heatexchanger, storage container, transfer line, pump, distilling apparatusor the like is stained with the polymer, and so problems such asincrease in the cost of washing, lowering in production efficiency andloss of material occur. The problems relating to such prematurepolymerization will be described more specifically taking the case of apurification step of a conjugated diene.

Conjugated dienes, conjugated diene-containing hydrocarbon mixtures (forexample, C₄ hydrocarbon fraction and C₅ hydrocarbon fraction),conjugated diene-containing unsaturated olefin hydrocarbon mixtures(mixtures in a recovery step of olefin hydrocarbon compounds after gasand liquid cracking or decomposing processes), conjugateddiene-containing monomer mixtures (for example, monomer mixtures forproduction of synthetic rubbers) and the like are easy to undergopolymerization of the conjugated dienes and/or copolymerization of theconjugated dienes with any other copolymerizable unsaturated compoundupon distillation, extractive distillation, extraction, countercurrentextraction, hydrogenation or hydrotreating, hydrorefining, heattreatment, other similar treatments, preheating before treatment,storage, transfer or processing.

For example, when a purified conjugated diene is isolated and recoveredfrom a conjugated diene-containing hydrocarbon mixture by a distillationprocess including extractive distillation, a polymerization reaction iseasy to occur to form a solvent-soluble polymer or a crosslinked,solvent-insoluble polymer. The solvent-soluble polymer is called arubbery polymer and stains various apparatus or devices such as anextractive distillation column, distillation column, heat exchanger andpiping. The crosslinked, solvent-insoluble polymer is a porous insolublepolymer and is called a popcorn polymer due to its appearance. Thispopcorn polymer is particularly undesirable because not only itself-multiplies in the presence of the vapor or liquid of the conjugateddiene to rapidly clog the apparatus, but it is extremely difficult toremove and control. Once the popcorn polymer is formed, it multiplies,so to speak, exponentially, in that it serves as a seed. Since thepopcorn polymer is a strong crosslinked polymer, it is insoluble in anyalready-known solvent and not melted. Accordingly, in order to removethe popcorn polymer, there is no effective cleaning method, but it iscleaned out by a mechanical means. The cleaning of the apparatusrequires to suspend and disjoint it so as to mechanically remove thedeposit of the polymer, and so it takes time, and economicaldisadvantage is unavoidable. In addition, since the popcorn polymercannot be completely removed by the mechanical cleaning, the popcornpolymer remaining in the apparatus in a trace amount serves as a seed tostart the multiplication of the popcorn polymer again when the operationof the apparatus is resumed.

In a process of preparing a purified conjugated diene by subjecting ahydrocarbon mixture containing the conjugated diene to a distillationprocess including extractive distillation, conditions liable to induce apolymerization reaction, such as coexistence of a gas phase with aliquid phase, moderate operating temperature, high monomer purity,mixing of water and presence of iron rust, gather. Accordingly, therehave heretofore been proposed methods making use of various kinds ofpolymerization inhibitors. However, in some cases, the insufficientpolymerization inhibiting effects thereof may have made it difficult toprevent the formation of a rubbery polymer and/or a popcorn polymer, sothat the apparatus is clogged. When the polymerization inhibitor is usedin a large amount to enhance the polymerization inhibiting effect, thereoccur such problems that a tar-like product is formed to waste energy,and the extraction efficiency of the extractive distillation is lowered.

In a process of recovering an olefin hydrocarbon compound such asethylene, propylene, butene, butadiene or a mixture thereof after gasand liquid cracking or decomposing processes, treatments such as anisolating process of various kinds of olefin hydrocarbon compounds areconducted by conversion by hydrogenation, distillation or extraction ofolefin compounds and acetylene compounds. Deposit (scale) considered tobe attributable to the polymerization of conjugated dienes and/or thelike is easy to form on apparatus for these treatments. When suchdeposit is built up to an excessive extent, thermal efficiency of theapparatus and isolation efficiency of a distillation column are lowered,and clogging of piping is caused. In addition, a monomer mixturecontaining a conjugated diene and a vinyl aromatic compound such asstyrene has been known to show a tendency to polymerize during itsstorage.

There has heretofore been proposed a method of distilling a C₅hydrocarbon fraction in the presence of N,N-dialkylhydroxylamine inorder to inhibit the polymerization of a conjugated diene-containingpetroleum fraction in a distillation apparatus (Japanese PatentApplication Laid-Open No. 112304/1975). However, the mere use ofN,N-dialkylhydroxylamine is not sufficient in thepolymerization-inhibiting effect. U.S. Pat. No. 3,371,124 has proposed amethod of using N,N-dialkyl-hydroxylamine and the oxalate[bis(diethylhydroxylamine) oxalate] thereof as polymerization inhibitorsin order to inhibit the formation of a popcorn polymer in a recoverysystem by fractional distillation of a monomer containing at least oneconjugated diene discharged from a production process of SBR. Theoxalate can be obtained by reacting N,N-dialkylhydroxylamine with oxalicacid. However, oxalic acid involves a problem that it corrodes anextractive distillation column. The mere use of the oxalate is notsufficient in the polymerization-inhibiting effect.

Japanese Patent Application Laid-Open No. 189810/1992 discloses a methodof causing [Group A] hydroquinone, hydroquinone monomethyl ether,p-methoxyphenyl, phenothiazine, piperidine, etc. and [Group B]phosphorus-containing compounds such as phosphoric acid and potassiumphosphate to coexist with a molecular oxygen-containing gas in order toinhibit thermal polymerization upon epoxidation of a double bond in acyclohexenyl ring by causing an epoxidizing agent to act on a mixture of(meth)acrylates having a cyclohexenyl group in their ester moieties.However, it has been found that when a combination of hydroquinone shownas a representative example of Group A in this publication with aphosphorus-containing compound in Group B is used to conduct extractivedistillation of a conjugated diene-containing hydrocarbon mixture, asufficient polymerization-inhibiting effect cannot be achieved.

Further, it has heretofore been proposed to use a compound having astable NO radical (free radical) in its molecule or a compound forming astable NO radical in situ under treating conditions as a polymerizationinhibitor (Japanese Patent Publication No. 26639/1992, etc.). However,the mere use of these compounds fails to achieve a sufficientpolymerization-inhibiting effect.

Such problems of premature polymerization are easy to occur at variousoperating steps in not only conjugated dienes, but also many othermonomers.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a polymerizationinhibitor and a method of inhibiting polymerization for inhibiting theoccurrence of unfavorable premature polymerization in various operatingsteps such as the production, purification, storage, shipment,preparation and use of a monomer such as a conjugated diene, aromaticvinyl, ethylenically unsaturated nitrile or α-olefin, a mixture of themonomers or a hydrocarbon mixture containing such a monomer.

Another object of the present invention is to provide apolymerization-inhibiting composition containing the monomer andpolymerization inhibitor described above.

A particular object of the present invention is to provide a novelpolymerization inhibitor, method of inhibiting polymerization andpolymerization-inhibiting composition containing a conjugated diene andthe polymerization inhibitor for inhibiting (co)polymerization of theconjugated diene upon the treatment, storage or the like of theconjugated diene, a conjugated diene-containing hydrocarbon mixture, aconjugated diene-containing unsaturated olefin hydrocarbon mixture, aconjugated diene-containing monomer mixture, or the like at a highlevel.

The present inventors have carried out an extensive investigation with aview toward overcoming the above-described problems involved in theprior art. As a result, it has been found that the combined use of atleast one compound selected from the group consisting of a compoundhaving an NO radical (NO) in its molecule and a precursor compoundcapable of forming an NO radical and a phosphorus-containing compound asa polymerization inhibitor is extremely effective for the inhibition ofpremature polymerization of various kinds of monomers such as conjugateddienes.

The polymerization inhibitor according to the present invention canmarkedly inhibit the formation of a popcorn polymer and a rubberypolymer when it is caused to exist in a distillation step in a processfor isolating and producing a purified conjugated diene by conducting adistillation process including extractive distillation from, forexample, a conjugated diene-containing hydrocarbon mixture. Thepolymerization inhibitor according to the present invention is extremelyeffective for not only hydrocarbon mixtures and the like containing aconjugated diene in a great amount, but also hydrocarbon mixtures andthe like containing a conjugated diene in a small amount. Thepolymerization inhibitor according to the present invention is alsoeffective for the inhibition of polymerization of monomers such asaromatic vinyls, ethylenically unsaturated nitrites and α-olefins. Thepresent invention has been led to completion on the basis of thesefindings.

According to the present invention, there is thus provided apolymerization-inhibiting composition comprising at least one compound(a) selected from the group consisting of a compound having an NOradical in its molecule and a precursor compound capable of forming anNO radical, a phosphorus-containing compound (b), and at least onemonomer (c) selected from the group consisting of a conjugated diene, anaromatic vinyl, an ethylenically unsaturated nitrile and an α-olefin,wherein a weight ratio of the compound (a) to the phosphorus-containingcompound (b) is 1:10 to 100:1.

According to the present invention, there is also provided apolymerization inhibitor for at least one monomer (c) selected from thegroup consisting of a conjugated diene, an aromatic vinyl, anethylenically unsaturated nitrile and an α-olefin, comprising at leastone compound (a) selected from the group consisting of a compound havingan NO radical in its molecule and a precursor compound capable offorming an NO radical, and a phosphorus-containing compound (b), whereina weight ratio of the compound (a) to the phosphorus-containing compound(b) is 1:10 to 100:1.

According to the present invention, there is further provided a methodfor inhibiting polymerization, which comprises causing at least onecompound (a) selected from the group consisting of a compound having anNO radical in its molecule and a precursor compound capable of formingan NO radical, and a phosphorus-containing compound (b) to coexist at aweight ratio of the compound (a) to the phosphorus-containing compound(b) of 1:10 to 100:1 with at least one monomer (c) selected from thegroup consisting of a conjugated diene, an aromatic vinyl, anethylenically unsaturated nitrile and an α-olefin.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram illustrating a distillation process includingan extractive distillation step for obtaining purified 1,3-butadienefrom a C₄ hydrocarbon fraction.

BEST MODE FOR CARRYING OUT THE INVENTION

1. Monomer

The polymerization-inhibiting technique according to the presentinvention can be applied to conjugated dienes, aromatic vinyls,ethylenically unsaturated nitrites and α-olefins. Examples of theconjugated dienes include 1,3-butadiene, isoprene, chloroprene and thelike. Examples of the aromatic vinyls include styrene, vinyltoluene,α-methylstyrene and the like. Examples of the ethylenically unsaturatednitriles include acrylonitrile, methacrylonitrile and the like. Examplesof the α-olefins include ethylene, propylene, 1-butene, 1-pentene,1-hexene, 1-heptene, 1-octene and the like.

Among others, the polymerization inhibitor according to the presentinvention is particularly effective for inhibiting the formation of apopcorn polymer and a rubbery polymer in a production process of apurified conjugated diene, comprising isolating the conjugated diene byconducting a distillation process including extractive distillation froma conjugated diene-containing hydrocarbon mixture. The conjugated dieneswill hereinafter be described in detail.

2. Conjugated Diene

In the present invention, as the conjugated dienes, may be mentioned1,3-butadiene, isoprene, chloroprene and the like. The polymerizationinhibitor according to the present invention is suitable for conjugateddienes or conjugated diene-containing mixtures. As the conjugateddiene-containing mixtures, may be mentioned conjugated diene-containinghydrocarbon mixtures, conjugated diene-containing unsaturated olefinhydrocarbon mixtures, conjugated diene-containing monomer mixtures andthe like.

The conjugated diene-containing hydrocarbon mixture is a mixture ofvarious kinds of hydrocarbons containing at least one conjugated diene.No particular limitation is imposed on a conjugated diene-containinghydrocarbon mixture used as a starting material for isolating andproducing a purified conjugated diene. However, as representativeexamples thereof, may be mentioned petroleum fractions such as anisoprene-containing C₅ hydrocarbon fraction and a1,3-butadiene-containing C₄ hydrocarbon fraction.

The C₅ hydrocarbon fraction is secondarily produced upon the productionof ethylene by steam cracking or another high-temperature treatment of ahydrocarbon. The C₅ hydrocarbon fraction generally has a boiling pointranging from 25° C. to 70° C. and contains various kinds of C₅hydrocarbons different in degree of saturation and may contain some C₄hydrocarbons and C₆ hydrocarbons. The C₅ hydrocarbon fraction generallycontains n-pentane, isopentane, 1-pentene, 2-methyl-1-butene,trans-2-pentene, cis-2-pentene, 2-methyl-2-butene, isoprene,trans-1,3-pentadiene, cis-1,3-pentadiene, 1,4-pentadiene, 2-butyne,isopropenylacetylene, isopropylacetylene, cyclopentane, cyclopentene,cyclopentadiene and the like.

The C₄ hydrocarbon fraction such as naphtha-cracked oil generallycontains various kinds of hydrocarbons such as propane, propylene,isobutene, allene, n-butane, isobutene, 1-butene, trans-2-butene,cis-2-butene, 1,3-butadiene, methylacetylene, 1,2-butadiene andvinylacetylene.

As the conjugated diene-containing unsaturated olefin hydrocarbonmixtures, may be mentioned unsaturated olefin hydrocarbon mixtures in arecovery step of olefin hydrocarbon compounds such as ethylene,propylene, butene, butadiene and mixtures thereof after gas and liquidcracking or decomposing processes. Ethylene, propylene, butene,butadiene and the like are isolated and recovered from these unsaturatedolefin hydrocarbon mixtures by a process including hydrogenation,distillation, extraction and/or the like.

As examples of the conjugated diene-containing monomer mixtures, may bementioned monomer mixtures for preparation of synthetic rubber such asSBR.

The polymerization inhibitor according to the present invention can beapplied to conjugated diene-containing liquid mixtures in addition tothe above-described mixtures.

3. Extractive Distillation

The polymerization inhibitor according to the present invention isparticularly suitable for use in inhibiting the formation of a popcornpolymer and a rubbery polymer in an extractive distillation step in aprocess for isolating and producing a purified conjugated diene byconducting a distillation process including extractive distillation froma conjugated diene-containing hydrocarbon mixture. Therefore, theextractive distillation will hereinafter be described in detail.

As the distillation process including extractive distillation, a singleextractive distillation process or a combination of plural extractivedistillation processes may be only conducted. However, when ahydrocarbon mixture containing many kinds of hydrocarbons is used as astarting material, an extractive distillation step may also be combinedwith a distillation step (fractional distillation step) making good useof a difference between boiling points.

As an example of the distillation process including extractivedistillation, may be mentioned a process for recovering high-purityisoprene from a C₅ hydrocarbon mixture described in Japanese PatentPublication No. 41323/1972. More specifically, this publicationdescribes a process for recovering high-purity isoprene, in which (1) aC₅ hydrocarbon mixture as a raw material is subjected to extractivedistillation in the presence of an N-alkyl-substituted lower fatty acidamide solvent containing a polymerization inhibitor to removehydrocarbons more hardly soluble than isoprene, (2) the thus-extractedisoprene and hydrocarbons more easily soluble than isoprene are thendistilled to remove most of cyclopentadiene and hydrocarbons having ahigher boiling point than isoprene, (3) the resultant fraction issubjected further to extractive distillation in the presence of theabove-described solvent to remove the remaining cyclopentadiene andeasily soluble hydrocarbons such as isopropenylacetylene, and (4) thesolvent before circulating to the extractive distillation is subjectedto a stripping treatment under such reduced pressure that the strippingtemperature amounts to 140° C. or lower. In the publication, a flowdiagram of the recovery process is illustrated and quoted for referencein explanation of the present invention.

As an example of a process for preparing (recovering) purified1,3-butadiene by a distillation process including extractivedistillation from a C₄ hydrocarbon fraction containing 1,3-butadiene,may be mentioned a process illustrated in FIG. 1. However, for example,reboilers, condensers, heat exchangers, coolers, pumps, circulatingcircuits in respective distillation columns, and the like are omitted inFIG. 1 for the sake of brief description of the whole distillationprocess.

As illustrated in FIG. 1, a gasified C₄ hydrocarbon fraction is fed to amiddle stage of a first extractive distillation column A through a pipe21, while an extraction solvent such as N,N-dimethylformamide is fedthereto through a pipe 45 to conduct first-stage extractivedistillation. In the first-stage extractive distillation, a raffinatecomposed of hydrocarbons (propane, propylene, isobutene, allene,n-butane, isobutene, 1-butene, trans-2-butene, cis-2-butene, etc.) lowerin solubility in the extraction solvent than 1,3-butadiene is removedfrom the top of the column through a pipe 22. The main component of theraffinate is butene. However, the gas discharged from the top of thecolumn is condensed by a condenser though not illustrated, and a part ofthe condensate is returned to the top of the column by refluxing. Thepressure within the first distillation column is generally 1 to 15 atm,and the temperature at the bottom of the column is generally 100 to 180°C. The number of plates in the extractive distillation column may besuitably preset and is generally 100 to 300 plates and often about 200plates in the case where the C₄ hydrocarbon fraction is used.

An extract containing 1,3-butadiene and hydrocarbons (methylacetylene,1,2-butadiene, vinylacetylene, etc.) higher in solubility in theextraction solvent than 1,3-butadiene is taken out of the bottom of thefirst distillation column A and fed to an upper part of a preliminarystripping column B through a pipe 23. In the preliminary strippingcolumn B, the hydrocarbons are partially stripped from the solvent anddirectly sent to a second distillation column E through a pipe 24. Thebottoms in the preliminary stripping column B are fed to the top of afirst stripping column C through a pipe 25, and the hydrocarbons arestripped from the solvent. The solvent discharged from the bottom of thefirst stripping column is cooled in a heat exchanger and circulated intothe first extractive distillation column A. A hydrocarbon vapordischarged from the top of the first stripping column is introduced intoa compressor D through pipes 26 and 27, compressed there and then fed tothe bottom of a second extractive distillation column E through a pipe28. The preliminary stripping column B and first stripping column C canbe operated under conditions that the pressure within each column isgenerally 1 to 2 atm, and the temperature at the bottom thereof is theboiling point of the solvent at the pressure thereof.

1,3-Butadiene and hydrocarbons higher in solubility in the extractionsolvent than 1,3-butadiene are mainly fed to the second extractivedistillation column E. An extraction solvent is fed to a position lowerby some plates than the top of the second extractive distillation columnE through a pipe 37. A vapor discharged from the top of the secondextractive distillation column is 1,3-butadiene containing a traceamount of impurities and refluxed by a condenser, and the remainingportion thereof is sent to a first fractional distillation column Hthrough a pipe 29. A liquid composed mainly of the solvent at the bottomof the second extractive distillation column E is first fed to abutadiene recovering column F through a pipe 33 and then to a secondstripping column G through a pipe 34, and the remaining hydrocarbons arestripped from the solvent there. The solvent discharged from the bottomof the second stripping column G is cooled by heat exchange and returnedto the first extractive distillation column A and second extractivedistillation column E through a pipe 36. A vapor at the top of thesecond stripping column G is refluxed by a condenser, and a gasremaining without being refluxed is discharged into a fuel gas systemthrough a pipe 35. The operating conditions of the second extractivedistillation column and second stripping column are the same as those ofthe first extractive distillation column and first stripping column,respectively.

Since a small amount of impurities still remain in a 1,3-butadienefraction even by the two-stage extractive distillation, these impuritiesare removed by fractional distillation. In the first fractionaldistillation column H, impurities having a boiling point lower than thatof 1,3-butadiene are removed. A vapor at the top of the first fractionaldistillation column H is partially condensed and refluxed, and theremainder is sent to the fuel gas system. The bottoms in the firstfractional distillation column H are sent to a second fractionaldistillation column I though a pipe 30. A distillate from the secondfractional distillation column I is provided as a 1,3-butadiene productthrough a pipe 31. The bottoms in the second fractional distillationcolumn I are discharged as a waste liquid. The operating conditions ofthe respective fractional distillation columns are such that thepressure within each column is generally 1 to 15 atm, and thetemperature within the column is the boiling point of the intendedproduct at the pressure thereof. The number of plates in eachdistillation column may be suitably preset and is generally 50 to 200plates and often about 100 plates in the case where the C₄ hydrocarbonfraction is used.

The extraction solvent is sent to a solvent-purifying column J, and thesolvent purified by washing with water is returned to the extractivedistillation columns though a pipe 44. Water and a waste liquid aredischarged through a pipe 40 and out of a pipe 41 for water and out of apipe 43 for the waste liquid.

In order to isolating and recovering a conjugated diene from the C₅hydrocarbon fraction and C₄ hydrocarbon fraction, as described above,there is adopted a distillation process in which {circle around (1)} atwo-stage extractive distillation step with the object of removinghardly soluble hydrocarbons and easily soluble hydrocarbons and {circlearound (2)} a fractional distillation step of generally two stagesmaking good use of a difference between boiling points, which isconducted between the extractive distillation steps of two stages or asa final step, are suitably combined with each other.

As the extraction solvent, is used any of various solvents which candissolve and extract conjugated dienes and generally used in a technicalfield relating to extractive distillation, such as amide compounds,N-methylpyrrolidone, acetonitrile and N-formylmorpholine. Among theseextraction solvents, amide compounds are preferred. As examples of theamide compounds, may be mentioned formamide, N,N-dimethylformamide,acetamide, N-ethylacetamide, N,N-dimethylacetamide, N-chloroacetamide,N-bromoacetamide, diacetamide, triacetamide, propionamide, butylamide,isobutylamide, valeramide, isovaleramide, hexanamide, heptanamide,octanamide, decanamide, acrylamide, chloroacetamide, dichloroacetamide,trichloroacetamide, glycol amide, lactamide, pyruvoamide,cyanoacetamide, 2-cyano-2-nitroacetamide, oxamide, malonamide,succinamide, adipamide, malamide, d-tartramide andN,N-dimethylacetoacetic acid amide. Among these, N,N-dimethylformamide(DMF) is particularly preferred.

Examples of extraction solvents other than those described above includeacetone, methyl ethyl ketone, dioxane, isoprene cyclic sulfone,acetonitrile, alcohol, glycol, N-methylolamine, N-ethylsuccinimide,N-methyl-pyrrolidone, hydroxyethylpyrrolidone,N-methyl-5-methyl-pyrrolidone, 2-heptenone, morpholine,N-formylmorpholine, N-methylmorpholin-3-on, sulfolane, methylcarbitol,tetrahydrofuran, aniline, N-methyloxazolidone, N-methyl-imidazole,N,N′-dimethylimidazolin-2-on, methyl cyanoacetate, ethyl acetoacetate,ethyl acetate, dimethyl malonate, propylene carbonate, methylcarbitol,diethylene glycol monomethyl ether, dimethyl sulfoxide andγ-butyrolactone.

A proportion of the extraction solvent used is generally 100 to 1,000parts by weight, preferably 200 to 800 parts by weight per 100 parts byweight of the conjugated diene-containing hydrocarbon mixture. Theextraction solvent is fed into each extractive distillation column froma position higher than a position in the column to which the hydrocarbonmixture is fed.

4. Polymerization Inhibitor

In the present invention, a polymerization inhibitor composed of acombination of at least one compound (a) selected from the groupconsisting of a compound having an NO radical in its molecule and aprecursor compound capable of forming an NO radical, and aphosphorus-containing compound (b) is used as a polymerization inhibitorfor conjugated dienes.

Compound (a)

The compound (a) includes inorganic compounds and organic compounds. Asspecific examples thereof, may be mentioned the following variouscompounds:

(1) N,N-Dialkylhydroxylamines represented by the formula (I):

wherein R₁ and R₂ are independently a linear, branched or cyclic alkylgroup having 1 to 10 carbon atoms.

The number of carbon atoms in the alkyl group is preferably 1 to 6.Examples of the alkyl group include methyl, ethyl, propyl, isopropyl,butyl, pentyl and cyclohexyl groups. The N,N-dialkylhydroxylamine ispreferably N,N-diethylhydroxylamine (DEHA).

(2) Such nitroxyl compounds (compounds having an NO radical in theirmolecules) of sterically hindered amines as disclosed in Japanese PatentPublication No. 237065/1985.

The nitroxyl compound (also referred to as N-oxyl or nitroxide) is afree radical having an unpaired electron and represented by thefollowing formula (II):

wherein the nitrogen atom is bonded directly to 2 tetrasubstitutedcarbon atoms, E₁, E₂, E₃ and E₄ are independently an organic group, andT is an organic group required to form a 5- or 6-membered ring.

E₁, E₂, E₃ and E₄ are preferably methyl groups. As examples of thenitroxyl compounds, may be mentioned4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl,4-oxo-2,2,6,6-tetramethylpiperidine-1-oxyl,4-benzoyloxy-2,2,6,6-tetramethylpiperidine-1-oxyl,3-carbamoyl-2,2,5,5-tetramethylpyrrolidine-1-oxyl,N-(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl-ε-caprolactam,3-oxyl-2,2,4,4-tetramethyl-7-oxa-3,20-diazaspiro[5.1.11.2]heneicosan-21-on,4-aza-3,3-dimethyl-4-oxyl-1-oxaspiro[4.5]decane and2,4,4-trimethyl-2-phenyloxazolidine-3-oxyl.

(3) Hydroxylamine compounds corresponding to such nitroxyl compounds ofsterically hindered amines as disclosed in Japanese Patent ApplicationLaid-Open No. 237065/1985.

The hydroxylamine compound is a precursor compound capable of forming anNO radical (free radical) in situ in a system to which thepolymerization inhibitor has been added and represented by the followingformula (III):

wherein the nitrogen atom is bonded directly to 2 tetrasubstitutedcarbon atoms, E₁, E₂, E₃ and E₄ are independently an organic group, andT is an organic group required to form a 5- or 6-membered ring.

E₁, E₂, E₃ and E₄ are preferably methyl groups. As examples of thehydroxylamine compounds, may be mentioned1,4-dihydroxy-2,2,6,6-tetramethylpiperidine,4-benzoyloxy-1-hydroxy-2,2,6,6-tetramethylpiperidine,di(1-hydroxy-2,2,6,6-tetramethylpiperidin-4-yl) sebacate andN-(1-hydroxy-2,2,6,6-tetramethylpiperidin-4-yl)-ε-caprolactam.

(4) Such nitrogen oxides having a stable free radical, precursorcompounds capable of forming a stable free radical in situ, etc. asdisclosed in Japanese Patent Publication No. 26639/1992.

Examples of the nitrogen oxides having a stable free radical (freeradical existing for such a long period of time as being detectable in astatic system by an ordinary spectroscopic method; half-life: generallyat least 1 year) include di-t-butylnitroxide, piperidinyl-1-oxycompounds, pyrrolidine-1-oxy compounds and pyrroline-1-oxy compounds.Examples of the piperidinyl-1-oxy compounds include4-hydroxy-2,2,6,6-tetramethylpiperidino-1-oxy,4-oxo-2,2,6,6-tetramethylpiperidino-1-oxy and2,2,6,6-tetramethylpiperidino-1-oxy.

Examples of the precursor compounds capable of forming a stable freeradical in situ include nitrones, nitrosos, thioketones, benzoquinonesand hydroxylamines. Nitrosophenylhydroxylamines and the ammonium saltsthereof may also be mentioned.

(5) Reaction products of an N,N-di-lower alkylhydroxyl-amine with anorganic acid disclosed in U.S. Pat. No. 3,371,124 and Japanese PatentPublication No. 17458/1966.

As examples of such reaction products, may be mentioned theabove-described oxalates [bis(diethyl-hydroxylamine) oxalate] ofN,N-dialkylhydroxylamines and the N,N-di-lower alkylhydroxylamine saltsof oxyacids or polycarboxylic acids described in Japanese PatentPublication No. 17458/1966.

The lower alkyl groups in the N,N-di-lower alkylhydroxylamines includemethyl, ethyl, propyl, isopropyl, butyl and hexyl groups, etc. Theorganic acids include oxalic acid, lactic acid,.tartaric acid, citricacid, malic acid, malonic acid, succinic acid, glutaric acid, adipicacid, pimelic acid, sebacic acid, azelaic acid, etc.

As specific examples of the N,N-di-lower alkyl-hydroxylamine salts, maybe mentioned diethylhydroxyl-ammonium citrate,bis(diethylhydroxylammonium) tartrate, bis(diethylhydroxylammonium)adipate and bis-dibutylhydroxylamine sebacate.

(6) N-Hydrocarbyloxy-substituted, hindered amine compounds disclosed inJapanese Patent Application Laid-Open No. 233905/1992; heterocycliccompounds such as phenothiazine, and primary, secondary or tertiaryhydroxylamine compounds disclosed in Japanese Patent ApplicationLaid-Open No. 233906/1992; N-oxycarbamoyl-substituted, hindered aminecompounds disclosed in Japanese Patent Application Laid-Open No.233907/1992; and N-OH compounds such asN-(1-hydroxy-2,2,6,6-tetramethylpiperidin-4-yl) caprolactam,bis(1-hydroxy-2,2,6,6-tetramethylpiperidin-4-yl) sebacate,1-hydroxy-2,2,6,6-tetramethylpiperidin-4-yl benzoate and1-hydroxy-2,2,6,6-tetramethylpiperidin-4-yl acrylate disclosed InJapanese Patent Application Laid-Open No. 288302/1992 may be used.

As typical compounds of these compounds, may be mentioned respectivecompounds represented by the formula (IV):

wherein the nitrogen atom is bonded directly to 2 tetrasubstitutedcarbon atoms, E₁, E₂, E₃ and E₄ are independently an organic group, andX is a divalent linking group; and the formula (V):

wherein the nitrogen atom is bonded directly to 2 tetrasubstitutedcarbon atoms, E₁, E₂, E₃ and E₄ are independently an organic group, andX is a divalent linking group.

In these formulae, E₁, E₂, E₃ and E₄ are preferably methyl groups. X ispreferably a divalent organic group such as —COO—(CH₂)_(n)—COO— (n=1 to20). As a preferable example of such a compound, may be mentionedbis(1-hydroxy-2,2,6,6-tetramethylpiperidin-4-yl) sebacate represented bythe following formula (VI):

(7) Nitrite

Nitrites such as sodium nitrite (NaNO₂) are inorganic precursorscompounds capable of forming an NO radical in situ in a system to whichthe polymerization inhibitor has been added. Sodium nitrite isparticularly effective in combined use with an inorganicphosphorus-containing compound such as sodium dihydrogenphosphate,phosphate type surfactant, tris(nonylphenyl)phosphite or the like whichwill be described subsequently.

Incidentally, among the above-mentioned compounds, the compounds (1) and(6) may overlap each other in some cases. Many of the above-describedcompounds are publicly known as agents for inhibiting prematurepolymerization of various kinds of monomers, or the like. However, theinvestigation by the present inventors have revealed that thesecompounds is not sufficient as polymerization inhibitor for variouskinds of monomers such as conjugated dienes. The feature of the presentinvention resides in that such a compound (a) is used in combinationwith a phosphorus-containing compound (b).

Phosphorus-containing Compound (b)

No particular limitation is imposed on the phosphorus-containingcompound (b). However, examples thereof include {circle around (1)}phosphoric compounds selected from the group consisting of phosphoricacid, phosphonic acid, phosphinic acid, diphosphonic acid,hypophosphoric acid, diphosphoric acid, tripolyphosphoric acid andmetaphosphoric acid; {circle around (2)} esterified products of thephosphoric compounds; {circle around (3)} alkali metal salts or ammoniumsalts of the phosphoric compounds; {circle around (4)} compoundsobtained by introducing an ester linkage and an alkali metal saltlinkage or an ammonium salt linkage into the phosphoric compounds;{circle around (5)} phosphine compounds; and {circle around (6)}hexaalkyl-phosphorus triamides. More specifically, as examples of thephosphorus-containing compounds, may be mentioned the followingcompounds:

<Phosphoric Compounds>

Phosphonic acid is a dibasic acid (left formula) of phosphorus having anoxidation number of 3 having a P—H linkage. The presence of a tautomer(right formula) may also be considered.

Phosphinic acid is a monobasic acid (left formula) of phosphorus havingan oxidation number of 1. The presence of a tautomer (right formula) mayalso be considered.

(7) Tripolyphosphoric acid (triphosphoric acid)

H₅P₃O₁₀  (7)

(8) Metaphosphoric acid

(HPO₃)_(n)  (8)

<Esterified products of phosphoric compounds>

R includes not only a hydrocarbon group such as an alkyl, phenyl oralkylphenyl group, but also a hydrophobic group, for example, analkylphenyl, polyethylene oxide or alkylphenylpolyethylene oxide group,or the like. Regarding this, the same shall apply to the followingcompounds.

Besides the above compounds, esters of various kinds of phosphoriccompounds, for example, phosphonates such as dimethyl phosphonate,diethyl phosphonate, triethyl phosphonate, triisopropyl phosphonate andtriphenyl phosphonate; and metaphosphates, are included.

The phosphonates are phosphorus-containing compounds represented by thefollowing formulae (12) to (14):

HP(O)(OH)(OR)  (12)

HP(O)(OR)₂  (13)

P(OR)₃  (14)

Specific examples of the esterified products of the phosphoric compoundsinclude triphenyl phosphate represented by the formula (15):

and tris(nonylphenyl)phosphite represented by the formula (16):

(C₉H₁₉—C₆H₄—O)P  (16)

As the esterified products of the phosphorus-containing compounds havinga hydrophobic group, may be mentioned various kinds of phosphate typesurfactants. Among these, phosphate type surfactants generally used asrust preventives are particularly preferred because they combines aninhibitory effect on the formation of a popcorn polymer and a rubberypolymer with a rust-preventing effect. Specific examples of suchphosphate type surfactants include alkyl dihydrogenphosphatesrepresented by the formula (17):

wherein R¹ is an alkyl group having generally 7 to 18 carbon atoms,often 8 or 9 carbon atoms, and n is the average number of moles addedand generally 1 to 18, often 2 to 8, dialkyl hydrogenphosphatesrepresented by the formula (18):

wherein R¹ is an alkyl group having generally 7 to 18 carbon atoms,often 8 or 9 carbon atoms, and n is the average number of moles addedand generally 1 to 18, often 2 to 8, and phosphates represented by theformula (19):

[R¹—C₂H₄O_(n−1)—C₂H₄O]_(m)P(O)(OH)_(3−m)  (19)

wherein R¹ is an alkyl group having generally 7 to 18 carbon atoms,often 8 or 9 carbon atoms, n is the average number of moles added andgenerally 1 to 18, often 2 to 8, and m is an integer of 1 to 3. Morespecifically, examples of the phosphate type surfactants includephosphate mixtures (for example, Ratemul P-909, product of KaoCorporation) represented by the formula (20):

such as polyoxyethylene nonyl phenyl ether phosphates, and trialkylphosphates (for example, Perex RP, product of Kao Corporation)represented by the formula (21):

wherein n is 2 to 6.

<Alkali Metal Salts or Ammonium Salts of Phosphoric Compounds>

wherein M is an alkali metal such as Na or K, or an ammonium group. Thesame shall apply to the following salts.

Specific examples of the secondary phosphates include sodiumdihydrogenphosphate (Na dihydrogen-phosphate) represented by the formula(25):

Alkali metal salts or ammonium salts of various kinds of phosphoriccompounds other than the above may also be used. A mixed salt with analkali metal and an ammonium group bonded to the same phosphoriccompound may also be used.

Specific examples of the alkali metal salts of the other phosphoriccompounds include sodium metaphosphate represented by the formula (26):

wherein n is generally 3 to 16, and when n is 6, the compound is sodiumhexametaphosphate, and sodium tripolyphosphate represented by theformula (27):

In addition, examples of the alkali metal salts or ammonium salts of thephosphoric compounds include potassium phosphate, ammoniumhydrogenphosphate, potassium pyrophosphate, sodium pyrophosphate,potassium tripolyphosphate and sodium tripolyphosphate.

<Compounds Obtained by Introducing an Ester Linkage and an Alkali MetalSalt Linkage or an Ammonium Salt Linkage into Phosphoric Compounds>

In the formula, R is an alkyl group, phenyl group, alkylphenyl group orsuch a hydrophobic group as described above, or the like, and M is analkali metal or ammonium group. The same shall apply to the followingformulae.

Specific examples of these compounds include dipotassium monoalkylphosphates represented by the formula (30):

and potassium dialkyl phosphates represented by the formula (31):

Specific examples of these compounds also include potassium 2-ethylhexylpyrophosphate and sodium 2-ethyl-hexyl pyrophosphate

<Phosphine Compounds>

Examples of the phosphine compounds include triphenylphosphine[(C₆H₆)₃P] and triethylphosphine [(C₂H₅)₃P] represented by the formula(32):

(R)₃P  (32)

<Hexaalkylphosphorus Triamides>

As the phosphorus-containing compound, may be used a hexaalkylphosphorustriamide represented by the formula (33.):

When R in the formula is a methyl group, the compound ishexamethylphosphorus triamide.

As the phosphorus-containing compounds used in the present invention,the phosphoric compounds, alkali metal salts or ammonium salts of thephosphoric compounds, tris(nonylphenyl)phosphite andhexamethylphosphorus triamide are preferred, with the phosphoriccompounds and alkali metal salts or ammonium salts of the phosphoriccompounds being particularly preferred.

The compound (a) and the phosphorus-containing compound (b) used incombination as the polymerization inhibitor according to the presentinvention are both stable, excellent in solubility in hydrocarbonmixtures and extraction solvents and good in handling property becausethey are liquid in many cases. The polymerization inhibitor according tothe present invention is excellent in anticorrosion in many cases, andso it can also inhibit the formation of a popcorn polymer by iron rustin addition to the fact that it is hard to corrode apparatus.

A weight ratio of the compound (a) to the phosphorus-containing compound(b) is 1:10 to 100:1, preferably 1:1 to 80:1, more preferably 1:2 to70:1. If the proportions of the compound (a) and phosphorus-containingcompound (b) used are outside the above range, it is difficult to bringabout the synergistic effect of both compounds.

When the polymerization inhibitor according to the present invention isused in a process for isolating and purifying a conjugated diene from aconjugated diene-containing hydrocarbon mixture, examples of a methodfor using the polymerization inhibitor include {circle around (1)} amethod in which the polymerization inhibitor is added to the conjugateddiene-containing hydrocarbon mixture fed to extractive distillation,{circle around (2)} a method in which the polymerization inhibitor isadded to an extraction solvent, and {circle around (3)} a method inwhich the polymerization inhibitor is added to a mixture of thehydrocarbon mixture and the extraction solvent in an extractivedistillation column. The polymerization inhibitor is preferablycontinuously fed from the side of an extractive distillation column(first extractive distillation column) at a position higher than a platein the extractive distillation column, to which an extraction solvent isfed, or an inlet or outlet of a condenser provided on the top of theextractive distillation column. The polymerization inhibitor may also befed to a second extractive distillation column and other fractionaldistillation columns as needed. Feeding methods include such methodsthat a solution obtained by dissolving a prescribed amount of thepolymerization inhibitor in the hydrocarbon mixture or extractionsolvent is sprayed or added dropwise. The polymerization inhibitoraccording to the present invention may be used by adding it to amonomer, monomer mixture, monomer-containing hydrocarbon mixture or thelike. The polymerization inhibitor according to the present inventionmay be used either by mixing the compound (a) with the compound (b) andthen adding the resultant mixture or by separately adding the compound(a) and the compound (b).

The polymerization inhibitor according to the present invention is usedin an amount sufficient to inhibit premature polymerization. When thepolymerization inhibitor is used in, for example, a purifying process ofa conjugated diene by extractive distillation, the amount of thepolymerization inhibitor is generally 0.1 to 2,000 ppm, preferably 50 to1,000 ppm based on the total weight of the conjugated diene-containinghydrocarbon mixture and the extraction solvent. When oxygen is presentin plenty in the system upon the extractive distillation, a polymer isformed and deposited on a condenser and the like to contaminate theapparatus. Since the mixing of oxygen is unavoidable, it is preferredthat an oxygen concentration in a gas phase discharged from theextractive distillation column be controlled by, for example, adding anoxygen scavenger to the extraction solvent so as to be of the order ofgenerally 1 to 300 ppm, preferably 5 to 200 ppm, more preferably 5 to100 ppm.

The polymerization inhibitor according to the present invention may alsobe suitably used in a process for recovering an olefin hydrocarboncompound such as ethylene, propylene, butene, butadiene or a mixturethereof after gas and liquid cracking or decomposing processes. Morespecifically, in a process for separating an organic feed flowcontaining an olefin hydrocarbon compound selected from among ethylene,propylene, butene, butadiene and mixtures thereof, and other unsaturatedolefin compounds into a flow of the olefin hydrocarbon compound from thetop of the column and a flow containing the other unsaturated olefinhydrocarbon compounds from the bottom of the column, the polymerizationinhibitor according to the present invention is used by adding it intothe organic feed flow. A proportion of the polymerization inhibitoraccording to the present invention used is generally selected from arange of 0.1 to 2,000 ppm based on the weight of the organic feed flow.

The polymerization inhibitor according to the present invention may alsobe used by adding it to a monomer such as a conjugated diene, aromaticvinyl, ethylenically unsaturated nitrile or α-olefin, or a mixture ofthe monomers. Even in this case, a proportion of the polymerizationinhibitor used is generally selected from a range of 0.1 to 2,000 ppmbased on the weight of the monomer or the mixture of the monomers.

Other polymerization inhibitors, chain transfer agents, oxygenscavengers and the like may be used upon the use of the polymerizationinhibitor according to the present invention within limits not impedingthe objects of the present invention. The polymerization inhibitoraccording to the present invention may be used by mixing bothcomponents. However, both components may be separately added into suchvarious systems as described above.

EXAMPLES

The present invention will hereinafter be described more specifically bythe following Examples and Comparative Examples.

Example 1

A flow from the bottom of a first extractive distillation column, whichhad been obtained by subjecting a C₅ hydrocarbon fraction to extractivedistillation with DMF in the first distillation column, i.e., an extract(more soluble extract) containing isoprene and substances (easilysoluble hydrocarbons) higher in solubility in the solvent than isoprene,was used as “a conjugated-diene containing hydrocarbon mixture” toconduct the following experiment. The concentration of isoprene in theextract is about 15 wt. %.

A 100-ml pressure glass container was charged with 20 g of the extract,180 ppm in total of oxygen (added 3 times in an amount of 60 ppm every 8hours), iron pieces (used for evaluating corrosive property) and apolymerization inhibitor and closed to conduct a reaction at 125° C. for24 hours.

With respect to the polymerization inhibitor, N,N-diethylhydroxylamine(DEHA) was used as the compound (a). As the phosphorus-containingcompound, was used a phosphate mixture (Ratemul P-909, product of KaoCorporation) represented by the formula (20):

which is a phosphate type surfactant (rust preventive). The respectivecomponents of the polymerization inhibitor were added 3 times each in anamount of 480 ppm every 8 hours.

After the reaction, the contents were filtered through filter paper, anda product obtained by drying solids remaining on the filter paper wasregarded as “Polymer”, while residue obtained by drying a filtrate wasregarded as “High Boil”. Both products are polymers formed bypolymerization of isoprene, and “High Boil” is comparatively low inpolymerization degree and hence corresponds to a rubbery polymer. Theamounts of Polymer and High Boil were measured to calculate out theirweight proportions (wt. %) to the amount of isoprene. On the other hand,change in the iron pieces was observed to evaluate the degree ofcorrosion. The unit is mg/dm² day (weight loss, mg on corrosion for aday per 100 cm²). The results are shown in Table 1.

Comparative Examples 1 to 4

Respective reactions were conducted in the same manner as in Example 1except that the polymerization inhibitor was changed to theircorresponding kinds and amounts added shown in Table 1. However, inComparative Example 2, 2.5% furfural was first added. In ComparativeExamples 3 and 4, the respective components were added 3 times each inan amount of 480 ppm every 8 hours. The results are shown in Table 1.

TABLE 1 High Polymerization Polymer Boil Total Corrosion inhibitor (%)(%) (%) (mg/dm² · day) Ex. 1 DEHA/phosphorus- 0.07 0.25 0.32 0.0containing compound (each 480 ppm/time) Comp. Not added 1.00 0.08 1.081.5 Ex. 1 Comp. Furfural 0.32 0.16 0.48 3.0 Ex. 2 (2.5%/system) Comp.DEHA 0.70 0.11 0.81 0.0 Ex. 3 (480 ppm/time) Comp. DEHA/oxalic acid 0.060.27 0.33 25.5  Ex. 4 (1:2 mol) (480 ppm/time) (Note) (1) A DMF solutionof crude isoprene (more soluble extract) was used. (2) Oxygen = 60 ppm.(3) Reaction conditions = 125° C. × 24 hours. (4) DEHA:N,N-diethylhydroxylamine.

As apparent from the results shown in Table 1, the polymerizationreaction of isoprene is inhibited, and moreover no corrosion is causedwhen the polymerization inhibitor according to the present invention isused (Example 1). On the other hand, when no polymerization inhibitor isadded (Comparative Example 1), the formation of a polymer is marked.When furfural proposed in the prior art was used (Comparative Example2), the inhibitory effect on polymerization reaction was little, andcorrosive property was recognized though it was extremely slight. WhenDEHA is used by itself (Comparative Example 3), the inhibitory effect onpolymerization reaction is insufficient. When DEHA and oxalic acid areused in combination (Comparative Example 4), the inhibitory effect onpolymerization reaction is good, but corrosive property is recognized,so that it is expected to form a popcorn polymer due to iron rust.

Examples 2 to 5, and Comparative Examples 5 to 20

Respective experiments were conducted in the same manner as in Example 1except that {circle around (1)} a DMF solution of purified isoprene(isoprene concentration=15 wt. %) was used in place of the DMF solutionof crude isoprene, {circle around (2)} no iron piece was added, {circlearound (3)} the polymerization inhibitor was changed to theircorresponding kinds and quantity proportions shown in Table 2, {circlearound (4)} the concentration of oxygen was changed from 60 ppm to 180ppm, and {circle around (5)} the reaction conditions were changed from125° C. for 24 hours to 100° C. for 3 days. However, the respectivecomponents were first added to the system in their corresponding amountsshown in Table 2. Incidentally, in Comparative Example 5, neither oxygennor polymerization inhibitor was added. In other Examples andComparative Examples, oxygen was added in an amount of 180 ppm. Theresults are shown in Table 2.

TABLE 2 High Polymer Boil Total Polymerization inhibitor (%) (%) (%) Ex.2 DEHA (180 ppm)/phosphoric acid 0.04 0.06 0.10 (180 ppm) Ex. 3 DEHA(360 ppm)/phosphoric acid 0.01 0.08 0.09 (180 ppm) Ex. 4 DEHA (180ppm)/Na dihydrogen- 0.01 0.06 0.07 phosphate (180 ppm) Ex. 5 DEHA (360ppm)/Na dihydrogen- 0.01 0.06 0.07 phosphate (180 ppm) Comp. Not added(oxygen was also not 0.06 0.12 0.18 Ex. 5 added) Comp. Only oxygen (180ppm) was added 0.54 0.08 0.62 Ex. 6 Comp. Furfural (2.5%) 0.26 0.12 0.38Ex. 7 Comp. DEHA (180 ppm) 0.19 0.09 0.28 Ex. 8 Comp. Phosphoric acid(180 ppm) 0.41 0.07 0.48 Ex. 9 Comp. Phosphoric acid (360 ppm) 0.26 0.080.34 Ex. 10 Comp. Na dihydrogenphosphate 0.09 0.32 0.41 Ex. 11 (180 ppm)Comp. Na dihydrogenphosphate 0.13 0.28 0.41 Ex. 12 (360 ppm) Comp.Hydroquinone (180 ppm) 0.41 0.48 0.89 Ex. 13 Comp. TBC (180 ppm) 0.040.31 0.35 Ex. 14 Comp. BHT (180 ppm) 0.61 0.10 0.71 Ex. 15 Comp.Monoethanolamine (180 ppm) 0.38 0.40 0.78 Ex. 16 Comp. Hydroquinone (180ppm)/ 0.34 0.11 0.45 Ex. 17 phosphoric acid (180 ppm) Comp. TBC (180ppm)/phosphoric acid 0.18 0.06 0.24 Ex. 18 (180 ppm) Comp. BHT (180ppm)/phosphoric acid 0.35 0.07 0.42 Ex. 19 (180 ppm) Comp.Monoethanolamine (180 ppm)/ 0.38 0.10 0.48 Ex. 20 phosphoric acid (180ppm) (Note) (1) A DMF solution of purified isoprene was used. (2) Oxygen= 180 ppm. (3) Reaction conditions = 100° C. × 3 days. (4) DEHA:N,N-diethylhydroxylamine. (5) TBC: 4-t-butylcatechol. (6) BHT:2,6-di-t-butyl-4-methylphenol.

As apparent from the results shown in Table 2, it is understood thatwhen the polymerization inhibitors according to the present inventionare used (Examples 2 to 5), the polymerization reaction is markedlyinhibited. On the other hand, when no polymerization inhibitor is used(Comparative Examples 5 and 6), or the conventional polymerizationinhibitors or polymerization inhibitors having a composition outside therange according to the present invention are used (Comparative Examples7 to 20), no sufficient inhibitory effect on polymerization reactioncannot be achieved. Accordingly, it can be understood that when themethod according to the present invention is applied to an actualdistillation process including extractive distillation, an excellentinhibitory effect on the formation of a popcorn polymer and a rubberypolymer is brought about.

Example 6

Twenty grams of a dimethylformamide (DMF) solution (isopreneconcentration=15 wt. %) of purified isoprene (purity: 99.3%) were placedin a 100-ml pressure glass container, and 180 ppm in total of oxygen(added 3 times in an amount of 60 ppm every 8 hours), 90 ppm of sodiumnitrite and 90 ppm of sodium dihydrogenphosphate were charged therein.After the container was closed, a reaction was conducted at 100° C. for3 days. Oxygen is added for the purpose of accelerating the reaction.After the reaction, the proportions of Polymer and High Boil formed werecalculated out. The results are shown in Table 3.

Examples 7 and Comparative Examples 21 to 24

The same procedure as in Example 6 was followed except that the kind andamount added of the polymerization inhibitor were respectively changedas shown in Table 3. However, in Comparative Example 21, nopolymerization inhibitor was added, but only oxygen was added. InComparative Examples 22 to 24 and Example 7, 180 ppm of oxygen were alsoadded in the same manner as in Example 6. The results are shown in Table3.

TABLE 3 High Polymer Boil Total Polymerization inhibitor (%) (%) (%)Comp. Oxygen (180 ppm) 0.46 0.21 0.67 Ex. 21 Comp. Nadihydrogenphosphate 0.03 0.27 0.30 Ex. 22 (180 ppm) Comp. Na nitrite (90ppm) 0.02 0.14 0.16 Ex. 23 Comp. Na nitrite (180 ppm) 0.02 0.12 0.14 Ex.24 Ex. 6 Na nitrite (90 ppm)/Na 0.01 0.06 0.07 dihydrogenphosphate (90ppm) Ex. 7 Na nitrite (180 ppm)/Na 0.01 0.06 0.07 dihydrogenphosphate(180 ppm)

As apparent from the results shown in Table 3, it is understood thatwhen the compound (a) and the phosphorus-containing compound (b) wereused in combination as a polymerization inhibitor (Examples 6 and 7),the polymerization of isoprene was markedly inhibited compared with theresults of Comparative Examples 21 to 24.

Example 8

Twenty grams of a DMF solution (butadiene concentration=15 wt. %) ofpurified butadiene (purity: 99.2%) were placed in a 100-ml pressureglass container, and 180 ppm in total of oxygen (added 3 times in anamount of 60 ppm every 8 hours), 120 ppm of sodium nitrite and 120 ppmof sodium dihydrogenphosphate were charged therein. After the containerwas closed, a reaction was conducted at 100° C. for 3 days. After thereaction, the amounts of Polymer and High Boil formed were measured inthe same manner as in Example 1 to calculate out their proportions (wt.%) to the amount of butadiene. The results are shown in Table 4.

Comparative Examples 25 and 26

The same procedure as in Example 8 was followed except that the kind andamount added of the polymerization inhibitor were respectively changedas shown in Table 4. However, in Comparative Example 25, nopolymerization inhibitor was added, but only oxygen was added. InComparative Example 26, 180 ppm of oxygen were also added in the samemanner as in Example 8. The results are shown in Table 4.

TABLE 4 High Polymer Boil Total Polymerization inhibitor (%) (%) (%)Comp. Oxygen (180 ppm) 0.29 0.08 0.37 Ex. 25 Comp. Na nitrite (120 ppm)0.05 0.12 0.17 Ex. 26 Ex. 8 Na nitrite (120 ppm)/Na 0.01 0.08 0.09dihydrogenphosphate (120 ppm)

As apparent from the results shown in Table 4, it is understood thatwhen the compound (a) and the phosphorus-containing compound (b) wereused in combination as a polymerization inhibitor (Example 8), thepolymerization of butadiene was markedly inhibited compared with theresults of Comparative Examples 25 to 26.

Example 9

Twenty grams of a DMF solution (butadiene concentration=15 wt. %) ofpurified butadiene (purity: 99.2%) were placed in a 100-ml pressureglass container, and 180 ppm in total of oxygen (added 3 times in anamount of 60 ppm every 8 hours), 90 ppm of sodium nitrite and 90 ppm ofsodium dihydrogenphosphate were charged therein. After the container wasclosed, a reaction was conducted at 100° C. for 3 days. After thereaction, the proportions of Polymer and High Boil formed werecalculated out. The results are shown in Table 5.

Comparative Examples 27 and 28

The same procedure as in Example 9 was followed except that the kind andamount added of the polymerization inhibitor were respectively changedas shown in Table 5. However, in Comparative Example 27, nopolymerization inhibitor was added, but only oxygen was added. InComparative Example 28, 180 ppm of oxygen were also added in the samemanner as in Example 9. The results are shown in Table 4.

TABLE 5 High Polymer Boil Total Polymerization inhibitor (%) (%) (%)Comp. Oxygen (180 ppm) 0.29 0.06 0.35 Ex. 27 Comp. Na nitrite (90 ppm)0.04 0.09 0.13 Ex. 28 Ex. 9 Na nitrite (90 ppm)/Na 0.01 0.05 0.06dihydrogenphosphate (90 ppm)

As apparent from the results shown in Table 5, it is understood thatwhen the compound (a) and the phosphorus-containing compound (b) wereused in combination as a polymerization inhibitor (Example 8), thepolymerization of butadiene was markedly inhibited compared with theresults of Comparative Examples 27 to 28 even when the amounts of thesecompounds added were small.

Example 10

Twenty grams of a DMF solution (isoprene concentration=15 wt. %) ofpurified isoprene were placed in a 100-ml pressure glass container, and180 ppm in total of oxygen (added 3 times in an amount of 60 ppm every 8hours), 180 ppm of 4-oxo-2,2,6,6-tetramethylpiperidine-1-oxyl and 180ppm of sodium dihydrogenphosphate were charged therein. After thecontainer was closed, a reaction was conducted at 100° C. for 3 days.After the reaction, the proportions of Polymer and High Boil formed werecalculated out. The results are shown in Table 6.

Example 11 and Comparative Examples 29 to 32

The same procedure as in Example 10 was followed except that the kindand amount added of the polymerization inhibitor were respectivelychanged as shown in Table 6. However, in Comparative Example 29, nopolymerization inhibitor was added, but only oxygen was added. InComparative Examples 30 to 32 and Example 11, 180 ppm of oxygen werealso added in the same manner as in Example 10. The results are shown inTable 6.

TABLE 6 High Polymer Boil Total Polymerization inhibitor (%) (%) (%)Comp. Oxygen (180 ppm) 0.39 0.19 0.58 Ex. 29 Comp. Nadihydrogenphosphate 0.02 0.29 0.31 Ex. 30 (180 ppm) Comp.4-Oxo-2,2,6,6-tetramethyl- 0.37 0.06 0.43 Ex. 31 piperidine-1-oxyl (180ppm) Comp. 4-Hydroxy-2,2,6,6-tetramethyl- 0.22 0.06 0.28 Ex. 32piperidine-1-oxyl (180 ppm) Ex. 10 4-Oxo-2,2,6,6-tetramethyl- 0.02 0.060.08 piperidine-1-oxyl (180 ppm)/Na dihydrogenphosphate (180 ppm) Ex. 114-Hydroxy-2,2,6,6-tetramethyl- 0.01 0.05 0.06 piperidine-1-oxyl (180ppm)/Na dihydrogenphosphate (180 ppm)

As apparent from the results shown in Table 6, it is understood thatwhen the compound (a) and the phosphorus-containing compound (b) wereused in combination as a polymerization inhibitor (Examples 10 and 11),the polymerization of isoprene was markedly inhibited compared with theresults of Comparative Examples 29 to 32.

Example 12

Twenty grams of a DMF solution (isoprene concentration=15 wt. %) ofpurified isoprene were placed in a 100-ml pressure glass container, and180 ppm in total of oxygen (added 3 times in an amount of 60 ppm every 8hours), 180 ppm of nitrosophenylhydroxyamine ammonium salt and 180 ppmof sodium dihydrogenphosphate were charged therein. After the containerwas closed, a reaction was conducted at 100° C. for 3 days. After thereaction, the proportions of Polymer and High Boil formed werecalculated out in the same manner as in Example 1. The results are shownin Table 7.

Comparative Examples 33 and 35

The same procedure as in Example 12 was followed except that the kindand amount added of the polymerization inhibitor were respectivelychanged as shown in Table 7. However, in Comparative Example 33, nopolymerization inhibitor was added, but only oxygen was added. InComparative Examples 34 and 35, 180 ppm of oxygen were also added in thesame manner as in Example 12. The results are shown in Table 7.

TABLE 7 High Polymer Boil Total Polymerization inhibitor (%) (%) (%)Comp. Oxygen (180 ppm) 0.22 0.06 0.28 Ex. 33 Comp. Nadihydrogenphosphate 0.13 0.24 0.37 Ex. 34 (180 ppm) Comp.Nitrosophenylhydroxyamine 0.26 0.07 0.33 Ex. 35 ammonium salt (180 ppm)Ex. 12 Nitrosophenylhydroxyamine 0.09 0.05 0.14 ammonium salt (180ppm)/Na dihydrogenphosphate (180 ppm)

As apparent from the results shown in Table 7, it is understood thatwhen the compound (a) and the phosphorus-containing compound (b) wereused in combination as a polymerization inhibitor (Example 12), thepolymerization of isoprene was markedly inhibited compared with theresults of Comparative Examples 33 to 35.

Example 13

Twenty grams of a DMF solution (isoprene concentration=15 wt. %) ofpurified isoprene were placed in a 100-ml pressure glass container, and180 ppm in total of oxygen (added 3 times in an amount of 60 ppm every 8hours), 180 ppm of sodium nitrite and 180 ppm of sodiumdihydrogenphosphate were charged therein. After the container wasclosed, a reaction was conducted at 100° C. for 3 days.

After the reaction, the proportions of Polymer and High Boil formed werecalculated out in the same manner as in Example 1. The results are shownin Table 8.

Example 14

The experiment was conducted in the same manner as in Example 13 exceptthat Perex RP (sesquipolyethylene-2-ethylhexyl phosphate) produced byKao Corporation was used in place of sodium dihydrogenphosphate. Theresults are shown in Table 8.

Example 15

The experiment was conducted in the same manner as in Example 13 exceptthat tris(nonylphenyl)phosphite was used in place of sodiumdihydrogenphosphate. The results are shown in Table 8.

Comparative Example 36

The experiment was conducted in the same manner as in Example 13 exceptthat no polymerization inhibitor was added, but only oxygen was added.The results are shown in Table 8.

Comparative Example 37

The experiment was conducted in the same manner as in Example 13 exceptthat 180 ppm of sodium nitrite were added as a polymerization inhibitor.The results are shown in Table 8.

TABLE 8 High Polymer Boil Total Polymerization inhibitor (%) (%) (%)Comp. Oxygen (180 ppm) 0.41 0.08 0.49 Ex. 36 Comp. Na nitrite (180 ppm)0.03 0.13 0.16 Ex. 37 Ex. 13 Na nitrite (180 ppm)/Na 0.01 0.05 0.06dihydrogenphosphate (180 ppm) Ex. 14 Na nitrite (180 ppm)/Perex 0 0.060.06 RP (180 ppm) Ex. 15 Na nitrite (180 ppm)/tris- 0.01 0.07 0.08(nonylphenyl)phosphite (180 ppm)

As apparent from the results shown in Table 8, it is understood thatwhen the compound (a) and the phosphorus-containing compound (b) wereused in combination as a polymerization inhibitor (Examples 13 to 15),the polymerization of isoprene was markedly inhibited compared with theresults of Comparative Examples 36 and 37.

Example 16

Twenty grams of a DMF solution (isoprene concentration=15 wt. %) ofpurified isoprene were placed in a 100-ml pressure glass container, and180 ppm in total of oxygen (added 3 times in an amount of 60 ppm every 8hours), 180 ppm of N,N-dimethylhydroxylamine (DEHA), 3.6 ppm of sodiumdihydrogenphosphate and iron pieces were charged therein. After thecontainer was closed, a reaction was conducted at 100° C. for 3 days.After the reaction, the proportions of Polymer and High Boil formed werecalculated out in the same manner as in Example 1. The results are shownin Table 9.

Examples 17 to 21, and Comparative Examples 38 to 46

The reaction was conducted in the same manner as in Example 16 exceptthat the polymerization inhibitor was changed to their correspondingkinds and amounts added shown in Table 9. The results are shown in Table9.

TABLE 9 Oxygen DEHA Phosphorus-containing compound Polymer High BoilTotal (ppm) (ppm) Kind (ppm) (%) (%) (%) Comp. Ex. 38 — — Not added —0.09 0.08 0.17 Comp. Ex. 39 180 — Not added — 0.58 0.08 0.66 Comp. Ex.40 180 180 Not added — 0.27 0.06 0.33 Comp. Ex. 41 180 — Nadihydrogenphosphate 180 0.05 0.23 0.28 Comp. Ex. 42 180 —Hexamethylphosphorus triamide 180 0.12 0.07 0.19 Comp. Ex. 43 180 —Triphenylphosphine 180 0.20 0.36 0.56 Comp. Ex. 44 180 — Trimethylphosphonate 180 0.08 0.28 0.36 Comp. Ex. 45 180 — Triphenyl phosphonate180 0.48 0.09 0.57 Comp. Ex. 46 180 — Triethylphosphine 180 0.27 0.250.52 Ex. 16 180 180 Na dihydrogenphosphate 3.6 0.02 0.06 0.08 Ex. 17 180180 Hexamethylphosphorus triamide 18 0.04 0.04 0.08 Ex. 18 180 180Triphenylphosphine 18 0.02 0.07 0.09 Ex. 19 180 180 Trimethylphosphonate 18 0.06 0.04 0.10 Ex. 20 180 180 Triphenyl phosphonate 180.05 0.09 0.14 Ex. 21 180 180 Triethylphosphine 18 0.05 0.10 0.15 (Note)(1) DEHA: N,N-diethylhydroxylamine.

As apparent from the results shown in Table 9, it is understood thatwhen the compound (a) and the phosphorus-containing compound (b) wereused in combination as a polymerization inhibitor (Examples 16 to 21),the polymerization of isoprene was markedly inhibited compared with theresults of Comparative Examples 38 to 46. In addition, no corrosion ofthe iron pieces was observed in the respective Examples.

Example 22

Twenty grams of a DMF solution (isoprene concentration=15 wt. %) ofpurified isoprene were placed in a 100-ml pressure glass container, and180 ppm in total of oxygen (added 3 times in an amount of 60 ppm every 8hours), 180 ppm of 4-hydroxy-2,2,6,6-tetramethyl-piperidine-1-oxyl(HTPO), 180 ppm of hexamethylphosphorus triamide and iron pieces werecharged therein. After the container was closed, a reaction wasconducted at 100° C. for 3 days. After the reaction, the proportions ofPolymer and High Boil formed were calculated out in the same manner asin Example 1. The results are shown in Table 10.

Examples 23 to 25, and Comparative Examples 47 to 49

The reaction was conducted in the same manner as in Example 22 exceptthat the polymerization inhibitor was changed to their correspondingkinds and amounts added shown in Table 10. The results are shown inTable 10.

TABLE 10 Oxygen HTPO Phosphorus-containing compound Polymer High BoilTotal (ppm) (ppm) Kind (ppm) (%) (%) (%) Comp. Ex. 47 — — Not added —0.09 0.08 0.17 Comp. Ex. 48 180 — Not added — 0.57 0.08 0.65 Comp. Ex.49 180 180 Not added — 0.14 0.07 0.21 Ex. 22 180 180Hexamethylphosphorus triamide 180 0.00 0.02 0.02 Ex. 23 180 180Triethylphosphine 180 0.01 0.04 0.05 Ex. 24 180 180 Triphenylphosphine180 0.06 0.05 0.11 Ex. 25 180 180 Trimethyl phosphonate 180 0.06 0.060.12 (Note) (1) HTPO: 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl.

As apparent from the results shown in Table 10, it is understood thatwhen the compound (a) and the phosphorus-containing compound (b) wereused in combination as a polymerization inhibitor (Examples 22 to 25),the polymerization of isoprene was markedly inhibited compared with theresults of Comparative Examples 47 to 49. In addition, no corrosion ofthe iron pieces was observed in the respective Examples.

Example 26

After 500 ml of styrene were washed twice with 200 ml of 1N caustic sodaand then twice with 200 ml of water, zeolite (Zeolite A-3, product ofTosoh Corp.) was poured therein to dry styrene. In this example, thewashing and drying were respectively conducted by shaking the mixturecontaining styrene and the like for 2 to 3 minutes in a bottle.

Styrene purified above was taken out and charged into an ampoule. Atthis time, 50 ppm of N,N-diethyl-hydroxylamine (DEHA) and 50 ppm of Nadihydrogenphosphate were added. An air atmosphere was used as anatmosphere. After a reaction was conducted at 120° C. for 1 hour, thereaction mixture was dried to measure the amount of High Boil formed.The result is shown in Table 11.

Examples 27 and 28, and Comparative Examples 50 to 53

The reaction was conducted in the same manner as in Example 26 exceptthat the polymerization inhibitor was changed to their correspondingkinds and amounts added shown in Table 11. The results are shown inTable 11.

TABLE 11 NO radical compound or Na dihydrogen- precursor phosphate HighBoil Kind (ppm) (ppm) (%) Comp. Not added —  0 10.40 Ex. 50 Comp. DEHA50  0 5.71 Ex. 51 Ex. 26 DEHA 50 50 1.79 Comp. HTPO 50  0 0.44 Ex. 52Ex. 27 HTPO 50 50 0.17 Comp. BOTS 50  0 3.34 Ex. 53 Ex. 28 BOTS 50 502.33 (Note) (1) DEHA: N,N-diethylhydroxylamine. (2) HTPO:4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl. (3) BOTS:bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) sebacate.

As apparent from the results shown in Table 11, it is understood thatwhen the compound (a) and the phosphorus-containing compound (b) wereused in combination as a polymerization inhibitor (Examples 26 to 28),the polymerization of styrene was markedly inhibited compared with theresults of Comparative Examples 50 to 53.

Examples 29 and 30, and Comparative Examples 54 and 55

The experiment was conducted in the same manner as in Example 26 exceptthat the proportions of the compound (a) and Na dihydrogenphosphate usedwere changed as shown in Table 12, and the reaction was conducted at120° C. for 3 hours. The results are shown in Table 12.

TABLE 12 NO radical compound or Na dihydrogen- precursor phosphate HighBoil Kind (ppm) (ppm) (%) Comp. HTPO 500  0 1.63 Ex. 54 Ex. 29 HTPO 500500 0.31 Comp. BOTS 500  0 2.54 Ex. 55 Ex. 28 BOTS 500 500 0.40 (Note)(1) HTPO: 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl. (2) BOTS:bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) sebacate.

As apparent from the results shown in Table 12, it is understood thatwhen the compound (a) and the phosphorus-containing compound (b) wereused in combination as a polymerization inhibitor (Examples 29 and 30),the polymerization of styrene was markedly inhibited compared with theresults of Comparative Examples 54 and 55.

Industrial Applicability

According to the present invention, there are provided polymerizationinhibitors, a polymerization-inhibiting method andpolymerization-inhibiting compositions comprising a monomer and apolymerization inhibitor for inhibiting the occurrence of unfavorablepremature polymerization in various operating steps such as theproduction, purification, storage, shipment, preparation and use of amonomer such as a conjugated diene, aromatic vinyl, ethylenicallyunsaturated nitrile or α-olefin, a mixture of the monomers or ahydrocarbon mixture containing the monomer.

The polymerization inhibitors according to the present invention areparticularly effective for inhibiting the formation of a popcorn polymerand a rubbery polymer in a production process of a purified conjugateddiene, comprising isolating the conjugated diene by conducting adistillation process including extractive distillation from a conjugateddiene-containing hydrocarbon mixture.

What is claimed is:
 1. A polymerization-inhibiting compositioncomprising at least one compound (a) selected from the group consistingof an N,N-dialkylhydroxylamine, a sterically hindered nitroxyl compound,a sterically hindered hydroxylamine compound and inorganic nitratecompound, a phosphorus-containing compound (b), and a conjugateddiene-containing hydrocarbon mixture selected from the group consistingof an isoprene-containing C₅ hydrocarbon fraction and a1,3-butadiene-containing C₄ hydrocarbon fraction, wherein a weight ratioof the compound (a) to the phosphorus-containing compound (b) is 1:10 to100:1.
 2. The polymerization-inhibiting composition according to claim1, wherein the compound (a) is at least one compound selected from thegroup consisting of an N,N-dialkylhydroxylamine, a sterically hinderednitroxyl compound and a sterically hindered hydroxylamine compound. 3.The polymerization-inhibiting composition according to claim 2, whereinthe N,N-dialkylhydroxylamine is a compound represented by the formula(I):

wherein R₁ and R₂ are independently a linear, branched or cyclic alkylgroup having 1 to 10 carbon atoms.
 4. The polymerization-inhibitingcomposition according to claim 2, wherein the sterically hinderednitroxyl compound is a compound represented by the formula (II):

wherein the nitrogen atom is bonded directly to 2 disubstituted carbonatoms, E₁, E₂, E₃ and E₄ are independently an organic group, and T is anorganic group required to form a 5- or 6-membered ring, or a compoundrepresented by the formula (IV):

wherein the nitrogen atom is bonded directly to 2 disubstituted carbonatoms, E₁, E₂, E₃ and E₄ are independently an organic group, and X is adivalent linking group.
 5. The polymerization-inhibiting compositionaccording to claim 2, wherein the sterically hindered hydroxylaminecompound is a compound represented by the formula (III):

wherein the nitrogen atom is bonded directly to 2 disubstituted carbonatoms, E₁, E₂, E₃ and E₄ are independently an organic group, and T is anorganic group required to form a 5- or 6-membered ring, or a compoundrepresented by the formula (V):

wherein the nitrogen atom is bonded directly to 2 disubstituted carbonatoms, E₁, E₂, E₃ and E₄ are independently an organic group, and X is adivalent linking group.
 6. The polymerization-inhibiting compositionaccording to claim 1, wherein the phosphorus-containing compound (b) isat least one selected from the group consisting of phosphoric compounds,esterified products of the phosphoric compounds, alkali metal salts orammonium salts of the phosphoric compounds, compounds obtained byintroducing an ester linkage and an alkali metal salt linkage or anammonium salt linkage into the phosphoric compounds, phosphinecompounds, and hexaalkylphosphorus triamides.
 7. A method for inhibitingpolymerization, which comprises causing at least one compound (a)selected from the group consisting of a N,N-dialkylhydroxylamine, asterically hindered nitroxyl compound, a sterically hinderedhydroxylamine compound and inorganic nitrate compound, and aphosphorus-containing compound (b) to coexist at a weight ratio of thecompound (a) to the phosphorus-containing compound (b) of 1:10 to 100:1with a conjugated diene-containing hydrocarbon mixture selected from thegroup consisting of an isoprene-containing C₅ hydrocarbon fraction and a1,3-butadiene-containing C₄ hydrocarbon fraction.
 8. Thepolymerization-inhibiting method according to claim 7, wherein thecompound (a) is at least one compound selected from the group consistingof an N,N-dialkylhydroxylamine, a sterically hindered nitroxyl compoundand a sterically hindered hydroxylamine compound.
 9. Thepolymerization-inhibiting method according to claim 8, wherein theN,N-dialkylhydroxylamine is a compound represented by the formula (I):

wherein R₁ and R₂ are independently a linear, branched or cyclic alkylgroup having 1 to 10 carbon atoms.
 10. The polymerization-inhibitingmethod according to claim 8, wherein the sterically hindered nitroxylcompound is a compound represented by the formula (II):

wherein the nitrogen atom is bonded directly to 2 disubstituted carbonatoms, E₁, E₂, E₃ and E₄ are independently an organic group, and T is anorganic group required to form a 5- or 6-membered ring, or a compoundrepresented by the formula (IV):

wherein the nitrogen atom is bonded directly to 2 disubstituted carbonatoms, E₁, E₂, E₃ and E₄ are independently an organic group, and X is adivalent linking group.
 11. The polymerization-inhibiting methodaccording to claim 8, wherein the sterically hindered hydroxylaminecompound is a compound represented by the formula (III):

wherein the nitrogen atom is bonded directly to 2 disubstituted carbonatoms, E₁, E₂, E₃ and E₄ are independently an organic group, and T is anorganic group required to form a 5- or 6-membered ring, or a compoundrepresented by the formula (V):

wherein the nitrogen atom is bonded directly to 2 disubstituted carbonatoms, E₁, E₂, E₃ and E₄ are independently an organic group, and X is adivalent linking group.
 12. The polymerization-inhibiting methodaccording to claim 7, wherein the phosphorus-containing compound (b) isat least one selected from the group consisting of phosphoric compounds,esterified products of the phosphoric compounds, alkali metal salts orammonium salts of the phosphoric compounds, compounds obtained byintroducing an ester linkage and an alkali metal salt linkage or anammonium salt linkage into the phosphoric compounds, phosphinecompounds, and hexaalkylphosphorus triamides.
 13. Thepolymerization-inhibiting method according to claim 7, wherein thecompound (a) and the phosphorus-containing compound (b) are caused tocoexist with a conjugated diene in a preparation process of a purifiedconjugated diene, comprising isolating the conjugated diene byconducting a distillation process including extractive distillation froma conjugated diene-containing hydrocarbon mixture.
 14. Thepolymerization-inhibiting method according to claim 8, wherein thephosphorus-containing compound (b) is at least one selected from thegroup consisting of phosphoric compounds, esterified products of thephosphoric compounds, alkali metal salts or ammonium salts of thephosphoric compounds, compounds obtained by introducing an ester linkageand an alkali metal salt linkage or an ammonium salt linkage into thephosphoric compounds, and hexaalkylphosphorus triamides.
 15. Thepolymerization-inhibiting method according to claim 14, wherein thecompound (a) is at least one compound selected from the group consistingof an N,N-dialkylhydroxylamine, a sterically hindered nitroxyl compoundand a sterically hindered hydroxylamine compound.
 16. Thepolymerization-inhibiting method according to claim 15, wherein theN,N-dialkylhydroxylamine is a compound represented by the formula (I):

wherein R₁ and R₂ are independently a linear, branched or cyclic alkylgroup having 1 to 10 carbon atoms.
 17. The polymerization-inhibitingmethod according to claim 15, wherein the sterically hindered nitroxylcompound is a compound represented by the formula (II):

wherein the nitrogen atom is bonded directly to 2 disubstituted carbonatoms, E₁, E₂, E₃ and E₄ are independently an organic group, and T is anorganic group required to form a 5- or 6-membered ring, or a compoundrepresented by the formula (IV):

wherein the nitrogen atom is bonded directly to 2 disubstituted carbonatoms, E₁, E₂, E₃ and E₄ are independently an organic group, and X is adivalent linking group.
 18. The polymerization-inhibiting methodaccording to claim 15, wherein the sterically hindered hydroxylaminecompound is a compound represented by the formula (III):

wherein the nitrogen atom is bonded directly to 2 disubstituted carbonatoms, E₁, E₂, E₃ and E₄ are independently an organic group, and T is anorganic group required to form a 5- or 6-membered ring, or a compoundrepresented by the formula (V):

wherein the nitrogen atom is bonded directly to 2 disubstituted carbonatoms E₁, E₂, E₃ and E₄ are independently an organic group, and X is adivalent linking group.
 19. The polymerization-inhibiting methodaccording to claim 7, wherein the compound (a) is anN,N-dialkylhydroxylamine.
 20. The polymerization-inhibiting methodaccording to claim 19, wherein the N,N-dialkylhydroxylamine is acompound represented by the formula (I):

wherein R₁ and R₂ are independently a linear, branched or cyclic alkylgroup having 1 to 10 carbon atoms.
 21. The polymerization-inhibitingmethod according to claim 20, wherein the phosphorus-containing compound(b) is at least one selected from the group consisting of phosphoriccompounds, esterified products of the phosphoric compounds, alkali metalsalts or ammonium salts of the phosphoric compounds, compounds obtainedby introducing an ester linkage and an alkali metal salt linkage or anammonium salt linkage into the phosphoric compounds, phosphinecompounds, and hexaalkylphosphorus triamides.